Laminating adhesives are widely used in the manufacture of film/foil laminates. Among many such known systems, the use of polyurethane based laminating adhesives is preferred because of their many desirable properties including good adhesion, peel strength, heat seal strength and resistance to aggressive filling goods. Typically, an isocyanate-containing polyurethane prepolymer obtained by the reaction of excess diisocyanate with a polyether and/or polyester containing two or more active hydrogen groups per molecule is used in combination with a second component. The second component usually is a polyether polyol and/or polyester polyol. The two components are combined in a predetermined ratio and applied on one of the film or foil substrates and laminated to the second substrate. Application may be from a solution in a suitable solvent using gravure or smooth roll coating cylinders or from a solvent-free state using special application machinery.
Solventless laminating adhesives (adhesives that can be applied at 100% solids and that do not contain either organic solvents or water) have a distinct advantage in that they can be applied and run at very high line speeds. This is due to the fact that no organic solvent or water has to be removed from the adhesive by drying. Solvent- or water-based laminating adhesives are limited to the application speed that the solvent or water can be effectively dried in an oven.
Typical line speeds for solvent-based and water-based laminating adhesives are 300 to 600 feet per minute due to the drying restrictions. Solventless adhesives, on the other hand, can be applied at 900 to even 1500 feet per minute. Solventless laminating adhesives thus have a distinct advantage over solvent-based or water-borne adhesives.
In order that the proper coating weight of laminating adhesive is applied to the substrate, the adhesive must be “metered down” by transfer rolls to the application web or substrate. This is generally achieved by transferring the adhesive from a “puddle” between two rolls to a second and sometimes third or fourth roll before applying to the substrate. The subsequent transfer rolls turn at a speed higher than the former roll so that the adhesive is less on each subsequent roll. Since these rolls are rotating at speeds up to 1000 rpm's, incomplete transfer of the adhesive typically occurs with the formation of adhesive “droplets” that are released into the air around the metering rolls. These adhesive “droplets” are seen as aerosol droplets that are commonly called “adhesive mist”.
Adhesive misting is undesirable for several reasons. The adhesive droplets tend to collect on machinery and other surfaces to form coatings that need to be periodically removed or cleaned. The droplets also can clog or constrict air vents and other exhaust or ventilation equipment. If the adhesive mist contains materials which are hazardous to human health due to toxicity or allergic sensitization, care must be taken to avoid worker exposure to such materials by inhalation of the adhesive droplets or deposition of the droplets on exposed skin or mucous membranes. Also, misting of the adhesive results in a loss of the adhesive, leading to increases in the cost of operating a lamination line.